Large scale structure is an important and frequently neglected
source of systematic uncertainty
([Somerville et
al. (2004)])
6. Comparison of
the left-hand and right-hand panels of
Fig. 7 illustrates this point
powerfully; the broad features of the evolution of early-type
galaxy stellar mass density are discernable using the
30' × 30' Chandra Deep Field South alone, but
correction of this result for cosmic variance using
the other two COMBO-17 fields yields a significantly
more convincing picture. Yet, while this kind of
correction for cosmic variance may work when there is
significant overlap between populations of interest
(although it is debatable how far one should push such an idea),
it is not a priori clear that rarer and/or optically-obscured
phases of galaxy evolution such as AGN or IR-luminous
mergers will be well modelled with such techniques.
Furthermore, short-timescale
astronomical phenomena, such as AGN or galaxy mergers,
have lower number density and are potentially very strongly
clustered leading to large uncertainties from number
statistics and cosmic variance.
Yet, these phases of galaxy evolution, where
galaxies undergo important and potentially permanent transformations
in the cosmic blink of an eye, require HST-resolution
data in order to explore their physical drivers.

When HST could be viewed as an essentially endless resource,
a piecemeal approach was perfectly optimal: more and/or larger
HST fields could be justified on a case-by-case basis, depending
on the science goals of interest. Yet, faced with an
unclear future for HST, it is not obvious that this approach
is optimal. HST perhaps should be thought of as a fixed lifetime
experiment, where the primary goal could become the creation of
an archival dataset which will support 10-20 years of
top-class research. In the creation of such an archival dataset,
important questions will need to be addressed: availability of
resources will need to be balanced against number statistics
and cosmic variance, arguably at least 2 HST passbands
will be required to allow some attempt at morphological
k-correction, and deep multi-wavelength data
will be required for study of black hole accretion and obscured
star formation, naturally
driving the fields into one of a small number of low HI and cirrus
holes (see Papovich, these proceedings).

Acknowledgments

I wish to warmly thank the GEMS and COMBO-17 collaborations -
Marco Barden, Steven Beckwith, Andrea Borch, John Caldwell, Simon Dye,
Boris Häußler, Catherine Heymans,
Knud Jahnke, Martina Kleinheinrich, Shardha Jogee,
Daniel McIntosh, Klaus Meisenheimer, Chien Peng, Hans-Walter Rix,
Sebastian Sanchez,
Rachel Somerville, Lutz Wisotzki, and last but by no means least
Christian Wolf - for their permission to present some
GEMS and COMBO-17 results before their publication, for
useful discussions, and
for their friendship and collaboration. It is a joy to be part
of these teams. I wish also to thank Eelco van Kampen and his
collaborators for their efforts to construct mock COMBO-17 catalogs,
for their permission to share results from these catalogs
in this article, and for useful comments.
Chris Conselice, Emmanuele Daddi, Sadegh Kochfar, and Casey Papovich
are thanked for useful and thought-provoking discussions on some of the
topics discussed in this review.
This work is supported by the European
Community's Human Potential Program under contract
HPRN-CT-2002-00316, SISCO.

6 It is interesting to note that a ×
10 increase in area
in a single contiguous field gives only a × 2 reduction
in cosmic variance, because the various parts of the single
field are correlated with each other.
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